This invention relates to an infrared intruder detection system, and in particular to an infrared intruder detection system comprising first and second separate channels A and B. Each channel has a pyroelectric detector device responsive to infrared radiation. The radiation receiving areas of the devices are closely spaced. Each channel also includes level detector means responsive to a predetermined output level of one polarity produced by the respective pyroelectric detector device as a result of an infrared image moving thereacross. The level detector provides an output signal when the detector device output attains the predetermined level. The detection system further includes circuit means for monitoring output signals of the level detector means in both channels, and for providing an output when output signals are produced in both channels.
The output from an intruder detection system of this kind may be used to generate an alarm in response to the detection of an intruder. The intruder acts as a moving source of infrared radiation crossing the field of view of the pyroelectric detector devices, thus causing the pyroelectric detector devices to produce outputs whose magnitudes exceed the predetermined levels of the level detector means.
The system may be used for other movement sensing purposes, for example in remote switching applications for sensing the presence of a person in a room and responding thereto to switch lights on or off. The term "intruder" should therefore be construed accordingly.
Two, separate, channels are provided in this known system in order to reduce the risk of false triggering. Factors such as electrical noise within pyroelectric detector circuits, or external, non-intruder related infrared radiation sources may cause the pyroelectric detector circuits to produce spurious outputs which could give rise to a misleading output. By employing two channels, each having its own pyroelectric detector device, and by requiring that intruder indicative outputs from both detector devices are needed in order for this system to respond, the risk of false triggering is considerably reduced. This is because the likelihood of spurious, intruder simulating noise signals occuring in both channels in such a manner to produce system response is remote.
Thus, the two channel system allows an alarm to be generated, or a switching function to be performed, on the basis of signal information in both channels. Noise interference in one channel only will generally not induce a reaction from the system. A situation where, say, noise spikes occur in both channels simultaneously or almost simultaneously would be very unlikely in view of the random nature of such noise spikes.
Besides having some immunity from false triggering, it is desirable of course that an intruder detection system be able to respond reliably to the presence of an intruder.
While the known system has improved false triggering immunity compared with earlier known single channel systems, it has been found that, in use, it can still sometimes prove unreliable in successfully detecting intruders.